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Phenylephrine or neosynephrine is an α-adrenergic receptor agonist used primarily as a decongestant, as an agent to dilate the pupil, and rarely to increase blood pressure. It has the CAS number 61-76-7.
Uses
Decongestant
Phenylephrine is used as a decongestant, available as an oral medicine or as a nasal spray. Phenylephrine is not the most common over-the-counter (OTC) decongestant; oxymetazoline is a more common nasal spray. Pseudoephedrine has generally been a more common oral decongestant, although this is changing; pseudoephedrine's notoriety as a methamphetamine precursor has led governments around the world to restrict its sale.Oral phenylephrine is extensively metabolised by monoamine oxidase, an enzyme which is present in the stomach and liver. Therefore, compared to orally-taken pseudoephedrine, it has a reduced bioavailability, and is less effective as a nasal decongestant. Because phenylephrine is a direct selective alpha-adrenergic receptor agonist, it does not cause the release of endogeneous noradrenaline like pseudoephedrine does. Therefore, phenylephrine is less likely to cause side-effects like central nervous system stimulation, insomnia, anxiety, irritability and restlessness.
Some popular cold remedies containing phenylephrine: Canada's hot lemon Neocitran, Britain's Lemsip, and the United States' Alka-Seltzer Cold Effervescent formula and DayQuil Capsules.
As a nasal spray, phenylephrine is available in 1% and 1/2% concentrations. It suffers some rebound congestion effects, although to a lesser degree than oxymetazoline.
Mydriatic
Phenylephrine is used as an eye drop to dilate the pupil to facilitate visualization of the retina. It is often used in combination with tropicamide. Narrow angle glaucoma is a contraindication to phenylephrine use.Vasopressor
Phenylephrine is rarely used as a vasopressor to increase the blood pressure in unstable patients with hypotension. Such use is more common in anesthesia practices; phenylephrine is especially useful in counteracting the hypotensive effect of epidural and subarachnoid anesthetics.Side effects
The primary side effect of phenylephrine is hypertension. Patients with congestion and hypertension are typically advised to avoid phenylephrine containing products.
Pregnancy
Phenylephrine is in the FDA pregnancy category C.
Legal Issues
Pseudoephedrine and phenylephrine are both used as decongestants; and until recently, pseudoephedrine was much more commonly available in the United States. This is changing, as pseudoephedrine can be used in the illegal manufacture of methamphetamine while phenylephrine cannot.
Since 2004, phenylephrine has been increasingly marketed as a substitute. In addition to selling a variety of medicines containing pseudoephedrine under the name brand Sudafed, Pfizer introduced "Sudafed PE" containing phenylephrine. Phenylephrine has been off patent for a long time; there are many generic brands available.
References
Sympathomimetics | Topical decongestants | Decongestants | Over-the-counter substances
"Phenylephrine"